The 3rd Generation Partnership Project, 3GPP, Release 11 has introduced a mechanism for optimizing media transfer for a Session Initiation Protocol, SIP, session originated by a roaming Internet Protocol, IP, Multimedia Subsystem, IMS, user. This method has been analysed in the RAVEL work item within 3GPP (3GPP TR 23.850; Study on roaming architecture for voice over IP Multimedia Subsystem (IMS) with local breakout (‘RAVEL’)). The result of this study is included in 3GPP TS 23.228 v11.3.0. FIG. 1 of the present document shows a conceptual architecture for Optimized media routing for an originating voice over Long Term Evolution, VoLTE, session by a roaming subscriber.
The media optimization according to 3GPP Release 11 applies to the media routing between the Visited IMS network of A-party (i.e. calling subscriber) and the Home IMS network of the B-party (i.e. called subscriber). When the B-party is roaming outside its Home IMS network, then the user plane for the call will be routed from the Visited IMS network of the A-party via the Home IMS network of the B-party to the Visited IMS network of the B-party.
This media transfer is not fully optimized. For example, when the A-party and the B-party are both roaming in the same country, the media will be routed from the Visited IMS network of the A-party via the Home IMS network of the B-party to the Visited IMS network of the B-party, and vice versa. The media is routed from the visited country to the home country of the B-party and back to the visited country. This is a situation which seemingly uses more media routing resources than necessary and adds costs to roaming for VoLTE subscribers.